Adaptability of lettuce cultivars to planting in no-tillage
on straw of
Urochloa ruziziensis
Andréia Cristina Silva Hirata(1), Edson Kiyoharu Hirata(2) and João Augusto Rizzo Camara(2)
(1)Agência Paulista de Tecnologia dos Agronegócios, Polo Regional da Alta Sorocabana, Caixa Postal 298, CEP 19015-970 Presidente Prudente, SP, Brazil. E-mail: andreiacs@apta.sp.gov.br (2)Universidade do Oeste Paulista, Rodovia Raposo Tavares, Km 572, CEP 19067-175 Presidente Prudente, SP, Brazil. E-mail: hirataedson@hotmail.com, guto_k3@hotmail.com
Abstract – The objective of this work was to evaluate the adaptability of different lettuce (Lactuca sativa) cultivars grown in no-tillage on straw of ruzi grass (Urochloa ruziziensis), as well as its effects on soil temperature and weed control, in successive cultivations in the summer. The treatments were carried out in a randomized complete block design, with four replicates, in a split-plot arrangement (2x4), in which soil management (no-tillage on ruzi grass and convention tillage) was the main plot, and the different crisphead lettuce cultivars (Amanda, Solaris, Vanda, and Vera), the subplots. No difference was observed between cultivars for soil management. No-tillage reduced the soil temperature by up to 3.7°C and weed density by 3.7 times, in comparison with conventional tillage. Ruzi grass promoted a greater bed conservation. In comparison to with conventional tillage, the no-tillage management of lettuce cultivated on ruzi grass straw
resulted in a lower yield in the first crop due to excess straw; in a similar yield in the second crop; and in a
greater yield in the third crop. Lettuce cultivated on ruzi grass straw in no-tillage management results in a greater sustainability, in comparison with conventional tillage, in three successive plantings.
Index terms: Brachiaria, Lactuca sativa, soil conservation, soil temperature, vegetable crops, weeds.
Adaptabilidade de cultivares de alface ao plantio direto
sobre palhada de
Urochloa ruziziensis
Resumo – O objetivo deste trabalho foi avaliar a adaptabilidade de cultivares de alface (Lactuca sativa) ao plantio direto sobre palhada de capim-congo (Urochloa ruziziensis), assim como seu efeito sobre a temperatura do solo e o controle de plantas daninhas, em cultivos sucessivos no verão. Os tratamentos foram dispostos no delineamento experimental de blocos ao acaso, com quatro repetições, em arranjo de parcelas subdivididas (2x4), em que os manejos do solo (plantio direto sobre capim-congo e plantio convencional) constituíram a parcela principal, e as diferentes cultivares de alface tipo crespa (Amanda, Solaris, Vanda e Vera), as subparcelas. As cultivares não diferiram entre si quanto ao manejo do solo. O plantio direto reduziu a temperatura do solo em até 3,7ºC, e a densidade de plantas daninhas em 3,7 vezes, em comparação ao plantio convencional. O capim-congo promoveu a maior conservação dos canteiros. Em comparação ao plantio convencional, o plantio direto de alface sobre palhada de capim-congo resultou em produtividade
menor no primeiro cultivo, em razão do excesso de palha; em produtividade semelhante, no segundo cultivo;
e produtividade superior, no terceiro cultivo. O cultivo de alface em plantio direto sobre palhada de capim-congo resulta em maior sustentabilidade, em comparação ao plantio convencional, ao longo de três cultivos sucessivos.
Termos para indexação: Brachiaria, Lactuca sativa, conservação do solo, temperatura do solo, hortaliças, plantas daninhas.
Introduction
Great improvements have been observed in
no-tillage systems for large-scale crops in Brazil;
however, intensive soil disturbance predominates in vegetable crops (Oliveira et al., 2006). Concerns over soil degradation have encouraged some countries
to decrease mechanical soil tillage operations (Price
& Norsworthy, 2013; Pieper et al., 2015). Due to the
evidence of excess soil tillage, the search for alternative
soil management systems has been intensified because
The growth of vegetable crops under no-tillage on straw of cover crops has been stimulated, aiming at the sustainability of natural resources in vegetable crop production. Researches have been implemented to increase the sustainability of the production of vegetable crops, such as lettuce (Hirata et al., 2014), green onion (Araújo Neto et al., 2010), and coriander (Tavella et al., 2010), among others, in no-tillage systems. Some studies have shown that different cultivars of a vegetable crop can differ for their adaptability to the same production system (Meagy et
al., 2013; Resende et al., 2016), and that it is important
to assess the compatibility of cultivars to a reduced soil tillage.
Another important aspect in vegetable crop
production is the difficulty of production in summer
conditions. Despite the advances of genetic breeding for tropically adapted lettuce by the development and release of cultivars more adapted to tropical conditions (Sala & Costa, 2012), lettuce grown in the summer period shows a decreased yield and early bolting
(Resende et al., 2007; Ferreira et al., 2009; Lafta et al., 2017); therefore, alternative production systems are
required for this planting period.
Because of the high-carbon/nitrogen ratio in their composition, grasses from the Urochloa genus are an option for straw production in no-tillage systems (Nepomuceno et al., 2012). Ruzi grass [Urochloa ruziziensis (Syn. Brachiaria ruziziensis)] also produces an excellent suppression effect for weed control (Silva Hirata et al., 2009). In general, a previous planting of grass suppresses populations of Rhizoctonia spp. and
Fusarium spp. in the soil (Toledo-Souza et al., 2008). However, further studies are required to identify appropriate cover crops and tillage systems, to increase sustainability in other agricultural settings such as vegetable crops (Price & Norsworthy, 2013).
The objective of this work was to evaluate the adaptability of lettuce cultivars grown in no-tillage management on straw of ruzi grass, as well as its effects on soil temperature and weed control in successive crops in the summer.
Materials and Methods
The experiment was carried out in the vegetable production sector of Polo Regional da Alta Sorocabana, from Agência Paulista de Tecnologia dos Agronegócios,
in the municipality of Presidente Prudente, in the state of São Paulo, Brazil (22°07'21"S, 51°23'17"W, at 460 m altitude), from September 2015 to April 2016.
According to the classification of Köppen-Geiger, the
predominant climate in the region is Aw, characterized by tropical rainy summers with dry winters. The coldest month shows an average temperature higher than 18°C, and the driest month shows precipitation lower than 60 mm (Ciiagro, 2016). The rainfall data during the experiment, as well as the maximum, mean, and minimum temperatures are presented in Figure 1.
The soil in the experimental area is classified as
an Argissolo Vermelho-Amarelo distrófico arênico (Arenic Hapludult). Soil chemical characteristics in the arable layer (0–20 cm) were determined according to the methods of Raij et al. (2001), as follows: pH (CaCl2)
6.0; soil organic matter, 14 g dm-3; base saturation,
86%; Ca, 45 mmolc dm-3; Mg, 18 mmolc dm-3; H + Al, 11 mmolc dm-3; Presin, 91 mg dm-3; K, 2.5 mg dm-3; Zn, 2.8 mg dm-3; Fe, 17 mg dm-3; Mn, 9.9 mg dm-3; Cu, 2.3 mg dm-3; and B, 0.17 mg dm-3. The soil has a sandy loam texture, containing 833 g kg-1 sand, 103 g kg-1 silt, and 64 g kg-1 clay.
The experimental design was a randomized complete block, with four replicates, in a split-plot arrangement. In the plots, the treatments were soil tillage (no-tillage on ruzi grass straw and conventional tillage), and, in the subplots, four lettuce cultivars (Amanda, Solaris, Vanda, and Vera).
The soil in the experimental area was prepared using a disc harrow and a leveling disc harrow. For raised beds of 1.2 m width and 0.30 m height, a rotary tiller was used. Ruzi grass was established by broadcasting seed in the plots on September 28, 2015, with 17 g m-2 of pure encrusted seed.
The chemical desiccation of ruzi grass was performed using glyphosate (1,440.0 g a.i. ha-1), on December 12, 2015, when forage was 65–70 cm high. Ruzi grass straw used in lettuce planting was at 7 Mg ha-1. The beds of conventional treatment plots were raised on December 18, 2015.
and potassium fertilizations were applied through fertigation, using half the recommended dose.
The lettuce seedlings (crisphead group) were produced in polypropylene trays with capacity for 288 seedlings. Lettuce seedlings were transplanted at 21 days after sowing, at 0.30x0.30 m spacing. The plantings were performed on December 26, 2015, February 6, 2016, and March 3, 2016. After each cultivation, the original plots were maintained, and glufosinate-ammonium was applied to dry the weeds. Water was supplied using a drip system, with three lines of drip-irrigation tubing arranged per plot. Drip holes were spaced at 0.30 m intervals.
The total plot area was 5.04 m2 – 4.2 m long and 1.2 m wide. Four rows of lettuce were planted in each plot, resulting in 56 plants per plot. The two central rows, that is, excluding the border rows, constituted the useful plot size. The plots were kept weed-free by hand hoeing.
Soil temperature was evaluated using type-K
thermocouple calibration sensors, with rigid stainless steel rods of 10 mm diameter, coupled to digital
thermometers. Temperature sensors were installed at 2.5 cm soil depth, in the central area of the plots. The temperatures were evaluated at 8:00 h and at 14:00 h. Evaluations were performed by inserting thermometers in all treatments of each block simultaneously. The soil temperature was accomplished in all lettuce crops, just before the lettuce canopy closed.
Weed samples were collected before hand-hoeing (15 days after planting) in each crop. Samples were collected by throwing a 0.30x0.30 m square quadrat at random, twice per plot. Weeds within the square
quadrat were identified, counted, and dried in a
forced-air oven at 65°C, until a constant weight was obtained. Chlorophyll index measurements were determined at harvest, using the Minolta SPAD-502 portable indirect chlorophyll meter (Minolta, 1989), in three lettuce growing seasons. Readings were taken from the ten fully expanded leaf, most recently developed per plot.
Ruzi grass straw was evaluated after each crop season, with samples collected by throwing a 0.30x0.30 m square quadrat at random, twice per plot.
Figure 1. Weather data for rainfall, and maximum, mean, and minimum air temperatures, during ruzi grass (Urochloa
The collected straw was dried in a forced-air oven at 65°C, until a constant weight was obtained. After the third crops, the bed height was evaluated.
At harvest, fresh weight was determined by cutting the plants close to the soil, just below the basal leaves. Senescent or diseased leaves were discarded, and lettuce yield (fresh weight) was evaluated. Head diameter, stem diameter, and number of leaves were
measured in five plants from the useful plot area.
Data were subjected to the analysis of variance, at 5% probability, and qualitative means were evaluated by the Tukey’s test, using the Assistat software (Silva, 2008). The quantitative means were evaluated by regression analyses.
Results and Discussion
The lettuce cultivar plots showed similar soil temperatures, irrespectively of soil management, which indicates that soil cover by canopy of cultivars was similar (Table 1). According to Oliveira et al. (2005), the temperature close to the soil surface is directly affected by its cover, which tends to intercept the sun rays before they strike the surface, resulting in
a specific microclimate under the straw.
The no-tillage treatment showed its highest temperature at 8:00 h, which indicates a lower-heat loss during the night. Blanco-Canqui et al. (2015) explain that cover crops control soil temperature by the increase of minimum temperature and the decrease of maximum
temperature. At 2:00 P.M., a temperature reduction of 3.7°C was observed in no-tillage management plots, in comparison with the conventional tillage. Horton et al. (1996) reported that straw has a higher albedo and lower-thermal conductivity than bare soil, reducing the solar energy reaching the soil.
In the second and third crops, there was no difference for soil temperature among soil management practices. A high reduction of the amount of straw at the end of the second crop (Figure 2), maximized by rainfall in this period, explained these results. Blanco-Canqui et al. (2015) reported that the amount of biomass strawresidues can determine the magnitude at which temperature is affected by a cover crop.
The cultivars did not differ for weed suppression, which is an evidence of a similar architecture and growth rate among cultivars. Beckie et al. (2008) found that weed suppression was positively correlated with canola and mustard biomass accumulation. However, Colquhoun et al. (2009) reported no difference among potato cultivars regarding the reduction of weed biomass.
High-weed infestation levels were observed in
the first crop (Table 2). In conventional tillage, weed
density and dry mass were 3.7 and 3.3 times greater, respectively, in comparison with no-tillage on ruzi
grass. There was no significant difference among treatments in the second crop; however, the total density was reduced in comparison with the first crop.
Table 1. Soil temperature at 8:00 A.M. and 2:00 P.M. time
points (hour) in different lettuce (Lactuca sativa) cultivars transplanted in no-tillage on straw of ruzi grass (Urochloa ruziziensis) or in conventional tillage, in successive crops(1).
Cultivar Soil-temperature (ºC) at 2.5 cm depth
Crop 1 Crop 2 Crop 3
8:00 2:00 8:00 2:00 8:00 2:00 Amanda 21.2a 29.5a 22.9 a 29.0a 22.2a 26.9a Solaris 21.4a 30.1a 22.7 a 29.5a 22.1a 26.7a Vanda 21.3a 29.7a 22.9 a 29.0a 21.9a 27.5a Vera 21.3a 29.1a 22.9 a 29.6a 22.2a 27.0a
CV (%) 1.5 3.0 0.9 2.0 1.7 2.4
Tillage
No-tillage 21.9a 27.8b 22.9 a 29.0 a 22.2a 26.8a Conventional 20.7b 31.5a 22.8 a 29.5 a 22.0a 27.2a
CV (%) 2.2 4.4 1.3 1.9 1.9 4.8
(1)Means followed by equal letters, in the columns, do not differ by Tukey’s test, at 5% probability.
Days after lettuce first crop
0 20 40 60 80 100 120
0 2 4 6 8
Y = 7.2938 - 0.0708x + 0.0003x2
R = 0.992
Ur
ochloa ruzizienis
straw (t ha
)
-1
Figure 2. Decomposition of ruzi grass (Urochloa
The conventional tillage resulted in higher values of weed density and total dry mass, in the third crop.
The reduction of weed density and dry mass in both management systems, observed in the second and third crops, occurred because even in the conventional tillage treatment the soil was not disturbed between the cultivations. Baskin & Baskin (2006) reported that soil disturbance brings seed to the soil surface, which exposes them to light and encourages the germination of nondormant species. Silva Hirata et al. (2009)
observed a high-weed control by ruzi grass in no-tillage tomato. According to Murphy et al. (2006), the reduction of soil tillage, in association with an adequate crop rotation, results in the decrease of weed density. Ruzi grass facilitated both functional soil cover and crop rotation.
The number of weed species was similar among the soil tillage types, with 18, 13, and 17 species surveyed in
the first, second, and third crops, respectively (Table 3).
Monocotyledons were predominant, particularly
Table 2. Dry matter and total density of weeds in the cultivation of different crisphead lettuce (Lactuca sativa) cultivars transplanted and managed in no-tillage, on straw of ruzi grass (Urochloa ruziziensis) or in conventional tillage, in successive crops(1).
Cultivar Total dry matter (g m-2) Total density (plants m-2)
Crop 1 Crop 2 Crop 3 Crop 1 Crop 2 Crop 3
Amanda 4.1a 7.2a 0.7a 250.7a 199.3a 69.4a
Solaris 3.1a 5.9a 0.5a 209.0a 170.1a 44.4a
Vanda 5.6a 8.4a 0.8a 404.2a 141.0a 75.7a
Vera 5.9a 8.6a 0.4a 342.4a 168.0a 45.1a
CV (%) 63.7 63.0 61.4 49.7 39.8 56.5
Tillage
No-tillage 2.2b 4.4a 0.2b 126.7b 154.2a 29.2b
Conventional 7.2a 10.6a 1.0a 476.4a 185.1a 88.2a
CV (%) 60.2 73.7 58.1 46.5 25.4 59.3
(1)Means followed by equal letters, in the columns, do not differ by Tukey’s test, at 5% probability.
Table 3. Weed species sampled in an experimental area, in the cultivation of different crisphead lettuce (Lactuca sativa) cultivars transplanted and managed in no-tillage (NT), on straw of ruzi grass (Urochloa ruziziensis) or in conventional tillage (CT), in successive crops.
Species Crop 1 Crop 2 Crop 3
NT CT NT CT NT CT
plants m-2
Digitaria horizontalis 66.00 237.85 71.20 42.70 9.00 20.15
Eleusine indica 6.63 82.30 30.58 88.53 13.55 47.93
Cyperus spp. 34.73 68.05 18.75 17.00 1.75 7.30
Commelina benghalensis 8.33 21.88 19.10 28.83 3.50 6.60
Chenopodium album 0.35 40.98 2.78 0.70 0.35 3.48
Portulaca oleracea 1.40 7.98 7.65 2.45 1.05 1.75
Amaranthus spp. 2.45 5.23 0.70 1.05 0.00 1.05
Richardia brasiliensis 1.40 3.15 0.70 0.70 0.00 0.00
Indigofera hirsuta 0.00 3.15 0.00 2.45 0.00 0.00
Sida spp. 2.10 2.80 0.00 0.35 0.00 0.00
Chamaesyce hirta 0.35 0.00 2.10 0.00 0.00 0.00
Brachiaria decumbens 0.70 1.40 0.00 0.00 0.00 0.00
Euphorbia heterophylla 1.40 0.00 0.00 0.00 0.00 0.00
Panicum maximum 0.00 0.35 0.70 0.00 0.00 0.00
Bidens pilosa 0.00 1.05 0.00 0.00 0.00 0.00
Portulaca pilosa 0.70 0.00 0.00 0.00 0.00 0.00
Acanthospermum hispidum 0.00 0.00 0.00 0.35 0.00 0.00
Cenchrus echinatus 0.35 0.00 0.00 0.00 0.00 0.00
Solanum americanum 0.00 0.35 0.00 0.00 0.00 0.00
Digitaria horizontalis and Eleusine indica. A high density of Cyperus spp., Commelina benghalensis, and
Chenopodium album was also recorded. The no-tillage management achieved a high-weed suppression of E.
indica (6.63 plants m-2 in no-tillage, and 82.3 plants m-2 in conventional tillage) and C. album (0.35 plants m-2 in no-tillage, and 40.98 plants m-2 in conventional
tillage), in the first crop. These findings corroborate the
results found by Peachey et al. (2004), who reported a reduced emergence of Solanum sarrachoides from 77 to 99%, and Amaranthus powellii from 50 to 87%, in a no-tillage system, in comparison with conventional tillage.
Pannacci et al. (2017) reported the importance of no-chemical weed management strategies in minor crops because of the low availability, or absence, of herbicides for these vegetable crops. Thus, considering
the difficulty of finding selective herbicides for
broadleaf weed control in lettuce, this crop system offers the potential for high-labor savings for hand hoeing.
As to crop yield potential, a higher-lettuce fresh weight was recorded for conventional tillage than for
no-tillage, in the first crop (Table 4). In the second
crop, the lettuce production was similar among tillage managements. In the third crop, however, the no-tillage system was superior to conventional no-tillage. Jokela & Nair (2016) reported a higher-broccoli yield in the conventional tillage than in no-tillage, on rye
and hairy vetch in the first crop, with equal yields in
the second crop.
The lower-lettuce fresh weight recorded in the first
crop can be attributed to lettuce planted 14 days after the chemical desiccation of ruzi grass that resulted in more straw on the soil surface. The process of ruzi grass root and shoot degradation limited the growth of lettuce, a vegetable that is adapted to soil with more intense tillage and without mechanical resistance. Despite this fact, planting after a short time following
desiccation resulted in a high yield for this first year
period (302 g per plant). Constantin et al. (2007) reported etiolation of corn that was sown in a period close to desiccation time, in an area that had high-cover crop residues. According to the authors, the greater energetic losses in the initial crop cycle resulted in a reduction of crop development, as the effectiveness of desiccation, or “lodging” of the cover crop, can affect future crop development.
Yield losses have been verified in annual crops,
when their sowings were performed soon after grass
desiccation (Argenta et al., 2001; Corrêa et al., 2008).
Grasses provide the advantage of maintenance cover on the soil surface for a longer period because of their high-carbon/nitrogen ratio. However, during the initial decomposition, there is a tendency of nutrient immobilization, as their sources in the cover
crop residues are not sufficient for the microbial
decomposition community, mainly nitrogen, which results in the reduction of some nutrients for the crops (Teixeira et al., 2009). Despite the higher-straw amount
Table 4. Yield and yield components of different crisphead lettuce (Lactuca sativa) cultivars transplanted and managed in no-tillage, on straw of ruzi grass (Urochloa ruziziensis) or in conventional tillage, in sucessive crops.
Cultivar Yield components
HD NL SD SWF
Crop 1
Amanda 35.8a 21.7ab 2.5a 340.1a
Solaris 35.3a 19.9b 2.4a 303.7a
Vanda 34.5a 23.1a 2.6a 364.5a
Vera 35.2a 21.5ab 2.5a 341.5a
CV (%) 3.7 6.9 4.5 15.8
Tillage
No-tillage 34.6a 20.5a 2.4a 302.0b
Conventional 35.8a 22.6a 2.6a 372.9a
CV (%) 7.2 9.9 8.2 13.4
Crop 2
Amanda 35.8a 20.7b 2.4b 258.0ab
Solaris 35.1a 21.3b 2.4b 248.0b
Vanda 36.1a 25.7a 2.7a 291.5a
Vera 35.2a 21.4b 2.4b 259.0ab
CV (%) 3.4 5.9 3.5 11.3
Tillage
No-tillage 35.8a 23.6a 2.5a 265.3a
Conventional 35.3a 21.0b 2.4a 262.9a
CV (%) 7.8 3.3 2.7 12.3
Crop 3
Amanda 35.4a 23.8b 2.46b 281.8ab
Solaris 35.4a 24.3b 2.5b 280.5ab
Vanda 33.7a 29.6a 2.8a 312.2a
Vera 35.2a 23.5b 2.4b 259.9b
CV (%) 3.8 2.4 4.8 12.6
Tillage
No-tillage 35.8a 25.9a 2.6a 303.1a
Conventional 34.1a 24.7b 2.5a 264.1b
CV (%) 6.1 5.6 5.0 8.9
(1)Means followed by equal letters, in the columns, do not differ by Tukey’s
of ruzi grass in the first crop, the index SPAD (Soil
Plant Analysis Development) related to N was higher in the no-tillage management. In the second and third crops, there were no differences among treatments (Table 5). Nitrogen fertilizer provided once a week by fertigation probably explains these results. The fertilizer can be a source to supply the immobilization of N. Therefore, it is likely that the physical hindrance for the lettuce establishment provided by ruzi grass roots, associated with a higher-straw amount in the
first crop, caused a greater effect on lettuce plants than
possible N immobilization.
There were no differences among cultivars for head diameter. 'Vanda' was the cultivar with the highest number of leaves and greatest stem diameter. Fresh
weight was similar among cultivars in the first crop;
however, 'Vanda' showed a greater fresh weight than 'Solaris' in the second crop, and 'Vera' in the third crop (Table 4). These results can be attributed to the climatic characteristics, in each planting period that favored one, or other cultivar.
Bed height was initially of 35 cm, and showed a greater reduction in the conventional tillage than in the no-tillage treatment (raised 81 days before the conventional tillage bed) (Table 5). The high rainfall of the summer period (Figure 1) is an important factor for determining the highest-soil loss. Bed conservation can be attributed to ruzi grass roots, as well as to the shoots that reduce the impact of raindrops. According to Salton
& Tomazi (2014), the cultivation of plants that show abundant root systems, such as grasses, contributed to the formation and stability of soil macroaggregates, which resulted in a higher-soil resistance to erosion and compaction. O’Rourke & Petersen (2016) reported that conventional-till pumpkin plots lost nine times more soil than conservation-tilled treatments.
Conclusions
1. Ruzi grass [Urochloa ruziziensis (syn. Brachiaria ruziziensis)] used as cover crop for lettuce (Lactuca sativa), in no-tillage, results in soil-temperature decrease up to 3.7°C, and in weed density reduction of 3.7 times, in comparison with conventional tillage.
2. There are no differences among the lettuce cultivars Amanda, Solaris, Vanda, and Vera for their performance in conventional or no-tillage systems.
3. Lettuce grown in no-tillage on ruzi grass shows more sustainable crop yield and improved conservation of bed structure than lettuce grown in conventional tillage.
Acknowledgments
To Fundação de Amparo à Pesquisa do Estado de
São Paulo (Fapesp), for financial support.
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Table 5. Soil plant analysis development (SPAD) index and bed height in the cultivation of different crisphead lettuce (Lactuca sativa) cultivars transplanted and managed in no-tillage, on straw of ruzi grass (Urochloa ruziziensis) or in conventional tillage, in sucessive crops(1).
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Solaris 18.46a 16.08a 16.16b 16.1a
Vanda 19.01a 17.32a 18.12a 17.1a
Vera 18.94a 16.88a 15.74b 16.9a
CV (%) 7.3 13.5 7.0 9.3
Tillage
No-tillage 19.86a 17.09a 16.15a 18.6a Conventional 17.61b 16.31a 16.67a 14.0b
CV (%) 4.9 13.8 3.6 16.8
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